For private packages, you'd just make the repo private on Github and make sure you have auth details in your .netrc file. So you don't really need your own "registry". There is talk about supporting more than just Github too.
That's a rather centralized solution. There are many companies who host their own npm/bower registries, and they won't move to Duo.js until that's possible.
But if you're at that point you probably have your own private git hosting somewhere too (or just private repos somewhere else). When there is support added for different remotes you could just require from there instead of Github.
Point is, just use the git repos instead of needing that registry at all.
